Issue 4

As announced in the March 1971 issue of The Structural Engineer (page 112) the Council awarded the 1971 Drury Medal to Mr. Peter Ellis, an Associate-Memberaged 24. The design submitted by Mr. Ellis was for an elevated tank for supplying water to an exhibition site. As well as the Drury Medal, Mr. Ellis receives a premium of £100.

The paper discusses the problem of design of rafts resting on soft subgrades in which long-term deformations lead to a redistribution of reactions. Design moments and shears are calculated by assuming a uniform upthrust, and it is usual to analyse an inverted raft on column supports subjected to a uniformly distributed loading. Errors inherent in such an approach are discussed and an alternative method is suggested. This assumes an elastic subgrade composed of very soft springs giving prominence to raft stiffness and ensuring uniform distribution upthrust. The method automatically satisfies local and overall equilibrium. A comparison of the two approaches on a raft analysed by the author for consulting engineers demonstrates the differences in design moments predicted by the two methods. F. Sawko

Mr. Parkhouse has presented a simple model to represent a cellular deck without intermediate diaphragms. The model has then been analysed for various loadings to determine the corresponding transverse distribution coefficients based on deflexion and moment. The loadings and deflexions have been represented as a sinusoidal series.

In the January issue of The Structural Engineer, p. 28, there was a report on the damage to a 7-storey office block in Belfast caused by a bomb attack. This report has been interpreted by some engineers as an indictment against permanent wood-wool formwork, which is unfortunate.

Mr. J. S. Trentham: It was obvious that this job was going to be one of great interest from the very beginning although nobody envisaged that it would receive the Concrete Society Award for 1971.

Erskine Bridge, over the Clyde estuary, is a multispan road bridge with the dual two-lane carriageways, footpaths and cycle tracks carried on a continuous high yield steel box girder of trapezoidal cross-section. The main span, of 350m (1000 ft), is stayed by single cables over centre towers above the main Piers and anchored in the median. The bridge is notable for the economy of material: steelwork in the superstructure weighs only 298 kg/m2 (61 lb/ft2) of deck area and steelwork plus mastic road surfacing 393 kg/m2 (80 lblft2). The cost of the bridge has been correspondingly attractive: £124 per m2 (£11.5 per ft2) of deck area. O.A. Kerensky, W. Henderson and W.C. Brown